1. Field of the Invention
The present invention relates to an improved method and apparatus for machining by means of rotary ultrasonic grinding ceramic disks that are used as substrates for computer hard disks. In general, rotary ultrasonic machining (RUM) has the potential for high material removal rates (MRR) while maintaining low cutting pressures and causing little sub-surface damage, which in turn results in little strength reduction in the workpiece being machined. The present invention implements RUM in a novel manner using a diamond grinding tool on a horizontal spindle-type machine to provide rotary ultrasonic grinding of a rotating ceramic disk substrate. The resulting disks are machined faster than with conventional methods, with a high degree of straightness and parallelism and with few defects.
2. Description of the Related Art
Advanced ceramics are increasingly used for numerous commercial applications in the aerospace, automotive, electronics and cutting tools industry. The inherent mechanical and thermal properties of the advanced ceramics (such as chemical inertness, high strength and stiffness at elevated temperatures, high strength to weight ratio, high hardness, corrosion resistance, and oxidation resistance) result in superior performance, which in turn, translates to significant cost savings.
The very properties of ceramics that make them attractive from a product performance standpoint are also responsible for difficulties encountered in shaping or machining them to a precise size and shape. Studies have concluded that the machining cost can be as high as 90% of the total cost of some ceramic components (see Jahanmir et al., "Ceramic Machining: Assessment of Current Practice and Research Need in the United States", NIST Special Publication, p. 834 (1992)). Additionally, the machining or shaping process is often responsible for strength degradation of the ceramic material. This can increase the susceptibility of the ceramic components to sudden failure, therefore decreasing their reliability.
Conventional ceramic products are usually sintered powder compacts. During the sintering process, shrinkage of materials cannot be avoided, making machining necessary to obtain proper dimensions (see Anantha Ramu et al., "Machining Performance of Toughened Zirconia Ceramic and Cold Compact Alumina Ceramic in Ultrasonic Drilling", 20 Journal of Mechanical Working Technology pp. 365-75, (1989)). Machining at high material removal rates (MRR) is desirable for manufacturing efficiency. There is therefore a crucial need for the development of processes which are capable of relatively high material removal rates while producing relatively little surface and sub-surface damage to the ceramic parts.
One possible application of ceramics is in the computer hardware industry. Magnetic computer hard disks are commonly manufactured with an aluminum substrate. Aluminum substrate disks generally do not provide the structural rigidity, flatness, and smoothness required for certain computer hardware applications. Because of the high strength to weight ratio of ceramics, the disk industry is actively pursuing replacement of the aluminum substrate with a ceramic one. The primary concern is the high cost of machining ceramics, particularly because tolerances on straightness and parallelism in computer hard disks are held typically to plus or minus 5 .mu.m (0.0002 in.), which requires lower MRR and therefore longer machining cycles using conventional machining by a diamond grinding, lapping, and polishing process. Disk manufacturers therefore seek a more efficient ceramic machining process.
Reports of conventional ultrasonic machining appeared in literature in the early 1960's (see "Ultrasonic Drilling with a Diamond Impregnated Probe", Ultrasonics, 1-4 (January-March 1964)). Such machining turned out to be an attractive proposition for machining hard and brittle materials, especially glass and ceramics. The performance of the process was found to improve with increasing hardness of the workpiece material. However, the material removal rates of this process were low.
RUM has recently been adapted for drilling and face milling of ceramics. (See Tyrell, "Rotary Ultrasonic Machine", presented at SME's Nontraditional Machine Seminar, (January 1970); Prabhakar, "Machining Advanced Ceramic Materials Using Rotary Ultrasonic Machining Process", M.S. Thesis, University of Illinois at Urbana-Champaign (1992). See also, Pei et al., "Rotary Ultrasonic Machining for Face Milling of Ceramics", accepted for publication in International J. of Mach. Tools and Mfg. (1995)). The results have been very promising in terms of achieving high MRR with minimal surface damage. RUM processes, however, generally require the use of a large tool and keeping the workpiece stationary to complete the machining task. Milling processes typically require manually moving the workpiece under the tool; thus, resulting in longer machining cycle times.
Accordingly, there is a need to provide an accurate, higher speed method of machining ceramic disks for use as computer disks to narrow tolerances of straightness and parallelism.
Therefore it is an object of this invention to provide an accurate, relatively high speed method of machining ceramic disks to narrow tolerances of straightness and parallelism.
It is a further object of this invention to provide high speed machining of ceramic disks with minimal subsurface damage caused by machining.
It is another object of this invention to provide an ultrasonic machining method that does not cause tool glazing.
It is still another object of this invention to provide ultrasonic machining of ceramics without the use of an abrasive slurry.
It is also an object of this invention to provide a machine for accurate, high speed grinding of ceramic disks.
It is still a further object of this invention to provide a ceramic disk having narrow tolerances of straightness and parallelism.